Continuously operated chromatographic separation processes commonly employ a simulated moving bed method (SMB method), which is used in a variety of different applications. The SMB method may be continuous or sequential or comprise a combination of a continuous method and a sequential method. In the continuous SMB process, all fluid streams typically flow continuously. In the sequential SMB process, some of the fluid streams do not flow continuously. The sequential SMB process commonly comprises three basic phases: a feeding phase, an elution phase and a circulation phase. During the feeding phase, a feed solution and possibly also an eluant during a simultaneous eluting phase, is introduced into a predetermined column containing one or more partial packed beds, and simultaneously a product fraction or fractions are withdrawn. During the eluting phase, the eluant is introduced into a predetermined partial packed bed or predetermined partial packed beds, and during these phases two, three or even four product fractions are withdrawn. During the circulation phase all columns are connected into a loop, whereby no feed solution or eluant is supplied to the partial packed beds and no product fractions are withdrawn. However, circulation as such takes place during all three phases.
The continuous SMB process has been described, for example, in U.S. Pat. No. 2,985,589 (Universal Oil Prod. Co). In this process the mixture to be fractionated is introduced into one partial packed bed and an eluant is introduced into another partial packed bed, and two product fractions are withdrawn substantially simultaneously. U.S. Pat. No. 5,198,120 (Organo KK) describes a continuous SMB process in which the feed point is fixed. The feed is introduced sequentially once a cycle and simultaneously with the introduction of the feed a first extract fraction and raffinate are taken out from the system. The examples of this patent use a simulated moving bed consisting of eight packed columns linked with each other in series, and each column, including the feed column, having an equal packed bed height.
Sequential SMB processes are described in U.S. Pat. Nos. 4,332,623 (Mitsubishi Chem. Ind.), 4,379,751 (Samnatsu Kogyo) and 4,970,002 (Mitsubishi Kasei Tech. Eng.), for instance. FIG. 1 of the above-mentioned U.S. Pat. No. 4,332,623 (Mitsubishi Chem. Ind.) discloses an apparatus for carrying out said method, which apparatus has three unit packed beds, which may be of the same or different size. Furthermore, the above-mentioned U.S. Pat. No. 4,970,002 (Mitsubishi Kasei Tech. Ing.) discloses a chromatographic separation apparatus including two packed beds, which may be the same or different in terms of bed capacity or the volume of the packing material.
A sequential SMB process for the recovery of betaine and sucrose from beet molasses is described in U.S. Pat. No. 5,127,957 (Heikkilä, H. et al.). To increase the separation capacity, yields and fraction purities and fraction dry substance concentrations, SMB modes including two or more loops or two or more separation profiles have been developed. In U.S. Pat. Nos. 6,093,326 (Danisco Finland Oy) and 5,637,225 (Xyrofin Oy) SMB processes including multiple loops are described. U.S. Pat. No. 6,224,776 (Cultor Corp.) discloses a method for fractionating a solution into two or more fractions in a SMB process where the separation system comprises at least two separation profiles in the same loop.
WO 01/54790 (Amalgamated Res. Inc.) describes a column apparatus for a fluid processing system containing a shallow bed of material between fluid transporting fractals of large active surface area. In said apparatus, said shallow bed of material has been provided as a column having greater diameter than height.
However, it does not appear from any of the above-mentioned references that the design of the feed column would differ significantly from that of the other columns used in the process.
One problem associated with the above SMB process modes is the so-called viscous fingering phenomenon which in turn causes the tailing effect of the chromatograms. Another problem with these SMB processes is the random resin movement which, as being irregular and turbulent, blends the frontiers and decreases the separation capacity.
It has now been found in accordance with the present invention that by using a shallow resin bed compartment as the feed column the problems above can be overcome or alleviated. With the use of a shallow resin bed compartment as the feed column, the viscous fingering and the random resin movement can be reduced. This leads to an improved separation profile in the feed column, which permits improved performance of the later columns.